Some years back i played La Hieronyma in a concert and in talking about the piece mentioned that it was the earliest solo work specifying trombone. A gentleman in the audience came up to me after and corrected me, saying there existed one earlier. Unfortunately, I cannot for the life of me remember what work he said preceded it. Perhaps it's something that's come to light more recently that the source you cited? Or perhaps this guy was wrong. Anyway, I thought I'd toss that out there.

Thanks for your comment. It kind of gets into semantics, but he could have been referring to this one (which is also on the list)--1620—Milan, Italy: An improvisational treatise by Francesco Rognoni (also called Francesco Rognoni Taeggio), Selva di varii passaggi, includes a setting of Orlando di Lasso’s song, Susanne un jour. Marked “Modo di passegiar il violone over trombone alla bastarda,” it features rapid technical passages to be played on either violone or trombone (Baines, Brass 114; Guion, Short History; Herbert, Trombone 87).

Update: I just added about a dozen more embedded videos/recordings, bringing the total recordings to just under 200. There are too many really nice ones to mention, but highlights include Adam Woolf playing Divisions on Palestrina’s Pulchra es, Michael Mulcahy playing Leopold Mozart with the Chicago Symphony, the ensemble "Sacabuche" playing several "trombone motets," and some beautiful Monteverdi and Gabrieli recordings by the "Green Mountain" project.